ASCE Section 12.8.7 (P-Delta Effects)

[McEngr]

Just wanting to check with Kevo or someone in the know: it appears to me that The Seismic Design Manual for AISC proceeds with not checking section 12.8.7 of ASCE 7 for P-Delta. The second order effects are part of this analysis and if section 12.8.7 were actually checked, using the beta factors for amplified first order effects would not be required.

Any comments welcome! Thanks!

 

[kevo_55]

McEngr,

The MF examples actually do check the drift & stability coef, but the BF examples don't.

Checking this part of ASCE 7-05 is technically required, but is neglected based on the high rigidity (ie: lower lateral drift) of the BF seismic systems. Well, this is how I see it.

Anyone else have any thoughts?

 

[McEngr]

McEngr,
The MF examples actually do check the drift & stability coef, but the BF examples don't.

Checking this part of ASCE 7-05 is technically required, but is neglected based on the high rigidity (ie: lower lateral drift) of the BF seismic systems. Well, this is how I see it.

Anyone else have any thoughts?

I guess because the commentary section C3 of the AISC 341 requires a different stability check than ASCE 7, the "theta" equation 12.8-16 of ASCE 7 is over-written. Perhaps this is the correct approach?

On page 6.1-126, it says the following, "The following discussion pertains primarily to moment frames (FEMA, 2000a); although other systems where high lateral drifts may occur require a similar analysis..."

I wonder if that is what eq 12.8-16 is for?

 

[McEngr]

McEngr,
The MF examples actually do check the drift & stability coef, but the BF examples don't.

Checking this part of ASCE 7-05 is technically required, but is neglected based on the high rigidity (ie: lower lateral drift) of the BF seismic systems. Well, this is how I see it.

Anyone else have any thoughts?

I guess because the commentary section C3 of the AISC 341 requires a different stability check than ASCE 7, the "theta" equation 12.8-16 of ASCE 7 is over-written. Perhaps this is the correct approach?

On page 6.1-126, it says the following, "The following discussion pertains primarily to moment frames (FEMA, 2000a); although other systems where high lateral drifts may occur require a similar analysis..."

I wonder if that is what eq 12.8-16 is for?

Thanks for the feedback kevo. Because I've realized in practice that MF have much higher lateral drifts than BF, I have only checked this for moment frames, but I'm still dusting off the cobwebs...

 

[epitome1170]

Thanks for the feedback kevo. Because I've realized in practice that MF have much higher lateral drifts than BF, I have only checked this for moment frames, but I'm still dusting off the cobwebs...

I have only spent a few minutes looking into this, so forgive me if this does not make sense, but my thoughts:

AISC specifications (whether that is chapter C or Appendix 7) applies an amplification to the loads to check the STRENGTH of the members for the P-Delta (and P-little delta) affects. This strength check does not necessarily correspond to a stability check.

My opinion is that the theta limitations established by ASCE 7 is established for said stability checks.

Ultimately, I agree that in steel construction this would result in"Ok, By Inspection" for braced frames, but requires a check for the moment frames. However, ASCE 7 is used for more materials than just steel so for a different system it would need to be checked and carefully considered.

Thoughts?

 

[kevo_55]

Nice thoughts on this!

To be honest, with the ability for software to generate a 2nd order analysis these days, the stability coef simply becomes a check for something to be "stable enough" for seismic loading.

I would say that the ASCE 7 version as well as FEMA (AISC 341-05 C3-1) version both could be used for MF.

 

[epitome1170]

Nice thoughts on this!
To be honest, with the ability for software to generate a 2nd order analysis these days, the stability coef simply becomes a check for something to be "stable enough" for seismic loading.

I would say that the ASCE 7 version as well as FEMA (AISC 341-05 C3-1) version both could be used for MF.

I agree, MF's should be checked for both ASCE and AISC equations.

And as you said, with computer software many of the code checks are now just a matter of pushing a few buttons (obviously once you have verified that the program does what it says it does... a step many people take too lightly IMO).

 

[McEngr]

Nice thoughts on this!
To be honest, with the ability for software to generate a 2nd order analysis these days, the stability coef simply becomes a check for something to be "stable enough" for seismic loading.

I would say that the ASCE 7 version as well as FEMA (AISC 341-05 C3-1) version both could be used for MF.

I agree, MF's should be checked for both ASCE and AISC equations.

And as you said, with computer software many of the code checks are now just a matter of pushing a few buttons (obviously once you have verified that the program does what it says it does... a step many people take too lightly IMO).

Great discussion. I appreciate the responses. I agree with kevo on this... I will probably only check one or the other. I personally think ASCE 7 is an easier calculation, but will probably go the chapter C route when it comes to moment frames since AISC wrote it.

 

[epitome1170]

Nice thoughts on this!
To be honest, with the ability for software to generate a 2nd order analysis these days, the stability coef simply becomes a check for something to be "stable enough" for seismic loading.

I would say that the ASCE 7 version as well as FEMA (AISC 341-05 C3-1) version both could be used for MF.

I agree, MF's should be checked for both ASCE and AISC equations.

And as you said, with computer software many of the code checks are now just a matter of pushing a few buttons (obviously once you have verified that the program does what it says it does... a step many people take too lightly IMO).

Great discussion. I appreciate the responses. I agree with kevo on this... I will probably only check one or the other. I personally think ASCE 7 is an easier calculation, but will probably go the chapter C route when it comes to moment frames since AISC wrote it.

Perhaps, I misunderstood kevo, but my thought was that BOTH had to be checked. That the ASCE 7 was checked for stability and the AISC/FEMA was checked for strength.

 

[McEngr]

epitome,

Because I've gone through the analysis a few times in practice, I have found that most moment frames that satisfy AISC seismic provisions (even without considering the drift limits of the AISC 341 commentary) will fall well witin the limits established by ASCE 7. Both should absolutely be checked.